Papers by Keyword: Pressure Dependence

Abstract: The diffusion mechanism of H in metals and metal hydrides is studied particularly at high H₂ pressures. Thin films of Mg and Ti offer a convenient tool to quantify the atomic transport. We show how different parameters of hydrogenation affect the kinetics. At 200°C, the Pd-Mg interface is predominant and a linear regime of hydrogenation is observed, whereas at 300°C a parabolic regime is detected. In Mg, the hydride forms from the surface to the substrate whereas in Ti growth of TiH₂ starts from the substrate. A linear kinetics is seen during hydrogenation of Ti films, which is due to the oxide layer on top, measured to be about 10nm thick. In the studied high pressure regime, the hydrogenation is not pressure dependent any more. Quantitative calculation of the growth rate and the diffusion coefficient of H in the hydrides is presented.

Abstract: In this Chapter, we review knowledge about diffusion in quasi-crystalline alloys (quasicrystals). In Section 1 we first remind the reader of some major aspects of the quasi-crystalline state and in Section 2 we introduce phase diagrams with quasi-crystalline phases, for which detailed diffusion studies are available. We mention in Section 3 the more common experimental methods for diffusion studies. The diffusive motion of atoms in quasi-crystalline alloys can be studied by the same techniques used for crystalline metallic alloys and intermetallics – measurements of radiotracer diffusion and diffusion of stable isotopes and solute atoms by SIMS profiling. The best-studied quasi-crystalline alloys are icosahedral AlPdMn, icosahedral ZnMgRE (RE = rare earth metal), and decagonal AlNiCo. The major diffusion results for these quasicrystals are reviewed in Sections 4, 5, and 6. Section 7 is devoted to the pressure dependence of diffusion in quasicrystals and to a comparison of the activation volumes with those of crystalline metals. Positron annihilation studies are also mentioned, which together with activation volumes for diffusion strongly favour a vacancy mechanism in quasicrystals. The major results and conclusions are summarized in Section 8.

Abstract: – Thermal contact resistances between a silver metallized SiC chip and a Molybdenum substrate and between the Molybdenum substrate and bulk Copper were measured in a heat transfer experiment. An experimental method to separate thermal contact resistances in a multilayer heat transfer path was used to extract the layer-specific contact resistances. The experimental results were compared with analytical calculations and also with 3-D computational fluid dynamics (CFD) simulation results. The results show significant pressure dependence of the thermal contact resistance and the results show higher thermal contact resistance per unit area between the bulk SiC chip and Molybdenum than between Molybdenum and bulk Copper.

Abstract: In this Chapter we review knowledge about diffusion and cation conduction in oxide glasses. We first remind the reader in Section 1 of major aspects of the glassy state and recall in Section 2 the more common glass families. The diffusive motion in ion-conducting oxide glasses can be studied by several techniques – measurements of radiotracer diffusion, studies of the ionic conductivity by impedance spectroscopy, viscosity studies and pressure dependent studies of tracer diffusion and ion conduction. These methods are briefly reviewed in Section 3. Radiotracer diffusion is element-specific, whereas ionic conduction is not. A comparison of both types of experiments can throw considerable light on the question which type of ions are carriers of ionic conduction. For ionic conductors Haven ratios can be obtained from the tracer diffusivity and the ionic conductivity for those ions which dominate the conductivity.In the following sections we review the diffusive motion of cations in soda-lime silicate glass and in several alkali-oxide glasses based mainly on results from our laboratory published in detail elsewhere, but we also take into account literature data.Section 4 is devoted to two soda-lime silicate glasses, materials which are commonly used for window glass and glass containers. A comparison between ionic conductivity and tracer diffusion of Na and Ca isotopes, using the Nernst-Einstein relation to deduce charge diffusivities, reveals that sodium ions are the carriers of ionic conduction in soda-lime glasses. A comparison with viscosity data on the basis of the Stokes-Einstein relation shows that the SiO₂ network is many orders of magnitude less mobile than the relatively fast diffusing modifier cations Na. The Ca ions are less mobile than the Na ions but nevertheless Ca is considerably more mobile than the network.Section 5 summarizes results of ion conduction and tracer diffusion for single Na and single Rb borate glasses. Tracer diffusion and ionic conduction have been studied in single alkali-borate glasses as functions of temperature and pressure. The smaller ion is the faster diffusing species in its own glass. This is a common feature of all alkali oxide glasses. The Haven ratio of Na in Na borate glass is temperature independent whereas the Haven ratio of Rb diffusion in Rb borate glass decreases with decreasing temperature.Section 6 reviews major facts of alkali-oxide glasses with two different alkali ions. Such glasses reveal the so-called mixed-alkali effect. Its major feature is a deep minimum of the conductivity near some middle composition for the ratio of the two alkali ions. Tracer diffusion shows a crossover of the two tracer diffusivities as functions of the relative alkali content near the conductivity minimum. The values of the tracer diffusivities also reveal in which composition range which ions dominate ionic conduction. Tracer diffusion is faster for those alkali ions which dominate the composition of the mixed glass.Section 7 considers the pressure dependence of tracer diffusion and ionic conduction. Activation volumes of tracer diffusion and of charge diffusion are reviewed. By comparison of tracer and charge diffusion the so-called Haven ratios are obtained as functions of temperature, pressure and composition. The Haven ratio of Rb in Rb borate glass decreases with temperature and pressure whereas that of Na in Na borate glass is almost constant.Section 8 summarizes additional common features of alkali-oxide glasses. Activation enthalpies of charge diffusion decrease with decreasing average ion-ion distance. The Haven ratio is unity for large ion-ion distances and decreases with increasing alkali content and hence with decreasing ion-ion distance.Conclusions about the mechanism of diffusion are discussed in Section 9. The Haven ratio near unity at low alkali concentrations can be attributed to interstitial-like diffusion similar to interstitial diffusion in crystals. At higher alkali contents collective, chain-like motions of several ions prevail and lead to a decrease of the Haven ratio. The tracer diffusivities have a pressure dependence which is stronger than that of ionic conductivity. This entails a pressure-dependent Haven ratio, which can be attributed to an increasing degree of collectivity of the ionic jump process with increasing pressure. Monte Carlo simulations showed that the number of ions which participate in collective jump events increases with increasing ion content – i.e. with decreasing average ion-ion distance. For the highest alkali contents up to four ions can be involved in collective motion. Common aspects of the motion process of ions in glasses and of atoms in glassy metals are pointed out. Diffusion in glassy metals also occurs by collective motion of several atoms.Section 10 summarizes the major features of ionic conduction and tracer diffusion and its temperature and pressure dependence of oxide glasses.

Abstract: – Thermal contact resistances between a silver metallized SiC chip and a direct bonded copper (DBC) substrate have been measured in a heat transfer experiment. A novel experimental method to separate thermal contact resistances in multilayer heat transfer path has been demonstrated. The experimental results have been compared with analytical calculations and also with 3D computational fluid dynamics (CFD) simulation results. A simplified CFD model of the experimental setup has been validated. The results show significant pressure dependence of the thermal contact resistance but also a pressure independent part.

Abstract: Firstly, this paper reminds the reader of some basic facts about the glassy state, then of the various ways to produce amorphous metals with particular emphasis on the route of vitrification from the melt. Vitrification of an undercooled melt is the most important route from the viewpoint of the application of metallic glasses. We compare diffusion in some metallic glasses with related crystalline metals. Glassy metals, also called metallic glasses, comprise conventional [1] and bulk metallic glasses [2,3]. We remind the reader of the major experimental techniques for diffusion studies in metallic glasses. The paper then reviews our current understanding of diffusion in glassy metals (see also [4,5,6]), including conventional as well as bulk metallic glasses and undercooled melts. We cover the temperature dependence of diffusion in metallic glasses and discuss the spectrum of activation parameters of glassy metals and its difference to the corresponding one of crystalline metals. We mention the pressure dependence and the isotope effect and we discuss tracer diffusion and viscosity diffusion for a bulk metallic glass and its undercooled melt. Finally we mention computer simulations of atomic jump processes. The diffusion mechanism in metallic glasses differs from that in crystalline metals and involves thermally activated, highly collective (chain-like or caterpillar-like) diffusion jumps. Finally, we mention diffusion along shearbands in a plastically deformed glassy metal.

Abstract: Thermal contact resistances have been measured in an experiment emulating heat transfer from a SiC die to a cooled heat sink through a heat spreader and a DBC structure. The major surface-dependent parameters are the surface roughness, surface hardness, and planarity. The measured thermal contact resistances are in agreement with theoretical values. When investigating DBC copper surfaces a second interface between the bonded Cu to AlN has to be taken into account.

Abstract: There are a number of well-known empirical relations for diffusion in solids. For example the proportionality between the self-diffusion activation energy and melting point or between the entropy of the diffusion and the ratio of activation energy and the melting point (Zener rule) are perhaps the best known ‘rules of thumb’. We have shown earlier in our Laboratory, that these relations are direct consequences of the similarity of interatomic potentials seen by ions in solids. On the basis of this, similar relations were extended for impurity and self diffusion in binary solid alloys. In this paper, results for binary liquid mixtures will be reviewed. First a minimum derivation of the temperature dependence of the self-diffusion coefficient, D, is presented (minimum derivation in the sense that it states only that the reduced (dimensionless) D should be a universal function of the reduced temperature), using the similarity of interatomic potentials and dimensional analysis. Then the extension of this relation for determination of the pressure and composition dependence of the self-diffusion coefficients is described using pressure and composition dependent scaling parameters (melting point, atomic volume and mass). The obtained universal form (valid for binary liquid alloys) is very useful for the estimation of the temperature, composition and pressure dependence of the self-diffusion coefficients. Finally, the relation for the ratio of the impurity and self-diffusion coefficients is derived.

Abstract: The gas pressure and the types of ambient gas dependence of X-ray intensity were investigated for LiNbO3 single crystals polarized in the c-axis direction at pressures of approximately 1 to 30 Pa. The integrated X-ray intensity showed a local maximum value at the pressure Pmax. Pmax moved to the high-pressure side in the ambient with a large first ionization energy. Pmax was proportional to the Boltzmann factor using the first ionization energy of each ambient gas molecule. The X-ray intensity was approximated using the quadratic function, which was convex upward for the pressure. It was found that one of the causes of the decrease in X-ray intensity on the pressure side higher than Pmax was the adsorption of positive ions on the crystal electric surface.